As the starting material for a carbon for an electrode, carbonized plant and animal material such as lignite, brown coal, anthracite coal, coke, wood charcoal, coconut shell char; any kind of resin such as phenol resin, furan resin, vinylidene chloride copolymer, etc., which have been heated (dry-distilled) in an inert gas, and the like may be used.
Because carbonaceous materials are chemically inactive, they are used in a wide range of applications such as adsorption agents, catalysts, electrode materials, structural materials for use in machines, etc.; however, these applications are closely related to the structure of the carbon.
That carbon which is referred to as porous carbon has special effects due to the development of pores. For example, using the adsorption phenomena, there are mixture separation and refining actions. In addition, the carbon used in electrical double layer capacitors, the carbon used in lithium secondary batteries, and the like display electrochemical storage effects.
The structure of the carbonaceous material can take various forms depending on the starting material and the manufacturing method. Char and activated carbon obtained by activating char comprise microcrystalline carbon (crystallite), and carbon which takes on a chain structure. When the carbonaceous material is a nongraphitizing carbon, the crystallites take on a structure which is layered in a disorderly manner, and a wide range of pores, from micropores to macropores, are formed in the gaps between these crystallites.
The crystallites are layers of net planes of six membered carbon rings of several parallel layers, and graphite carbon which has a six membered carbon ring structure bonds using hybridized orbitals SP.sup.2. A net plane comprising six membered ring carbon is called a basal plane.
A graphitizing carbon develops crystallites by means of heating at a high temperature, and finally becomes graphite.
A nongraphitizing carbon and a graphitizing carbon which has not been completely graphitized usually contain unorganized carbon. Unorganized carbon refers to carbon other than graphite carbon which is chemically bonded to graphite carbon only; carbon which has a chain structure; carbon which is stuck around six membered ring carbon; carbon which is in the periphery (the prism plane) of six membered ring carbon; carbon which is held in cross-linked structures with other six membered carbon rings (crystallites), and the like. Unorganized carbon is bonded with oxygen atoms, hydrogen atoms, and the like in forms such as C--H, C--OH, C--OOH, and C.dbd.O; or is in the form of double bonded carbon (--C=C--).
Lithium secondary batteries which use porous carbonaceous material in the negative electrode are charged by means of the uptake (doping) of lithium ions by the carbonaceous material of the negative electrode and are discharged by the release (un-doping) of lithium ions. In this lithium secondary battery, the charging capacity is determined by the amount of lithium ions with which the carbonaceous material is doped and the discharging capacity is determined by the un-doping amount. The efficiency of the electrical charging and discharging is defined as the ratio of the charging capacity to the discharging capacity.
When using graphite as the above-mentioned carbonaceous material, the lithium ions are taken in between the layers of the net planes of the carbon. In this case, the opinion is that the theoretical maximum for the doping quantity is when there is one lithium ion for every six carbon atoms. However, there are reports that, when non-graphitizing carbonaceous material is used, charging capacities which exceed the above-mentioned theoretical maximum amount can be obtained.
To the present, various proposals have been made for manufacturing methods for electrode carbon for lithium secondary batteries. For example, those recited in Japanese Patent Application, First Publication, No. Hei 2-66856; Japanese Patent Application, First Publication, No. Hei 6-187972; Japanese Patent Application, First Publication, No. Sho 61-218060; Japanese Patent Application, First Publication, No. Hei 5-335017; Japanese Patent Application, First Publication, No. Hei 2-230660; Japanese Patent Application, First Publication, No. Hei 5-89879; Japanese Patent Application, First Publication, No. Hei 5-182668; Japanese Patent Application, First Publication, No. Hei 3-245473; and Japanese Patent Application, First Publication, No. Hei 5-144440.
Japanese Patent Application, First Publication, No. Hei 2-66856 discloses that a carbon in which the distance d.sub.002 of the crystals is 3.80 .ANG. and for which the density is 1.55 g/cm.sup.3 can be obtained by carbonizing furfuryl alcohol resin at 500.degree. C., and then heat-treating it at 1100.degree. C., and that it is possible to dope the spaces between the carbon net planes with a large amount of lithium ions.
Japanese Patent Application, First Publication, No. Hei 6-187972 obtains a carbonaceous material by reacting a condensed polynuclear aromatic compound and a cross linking agent such as paraxyleneglycol, and baking the generated resin at a temperature of 1000.degree. C. or greater. The aromatic component forms a crystallized graphite structure and the cross-linking agent forms a non-crystallized domain, and this is suitable as a carbonaceous material for a lithium secondary battery.
Japanese Patent Application, First Publication, No. Sho 61-218060 discloses that a substance obtained by heat-treating an aromatic condensed resin, such as polyacene, and which has a H/C atomic ratio of 0.5.about.0.05, a BET specific surface area of 600 m.sup.2 /g or greater, and communicating pores having an average pore size of 10 .mu.m is suitable. It discloses that it is possible to manufacture a carbon having the above-mentioned characteristics by means of adjusting an aqueous solution of an initial polymer and an inorganic salt such as zinc chloride, and then heat-treating this at a temperature of 350.about.800.degree. C. which causes a three dimensional network structure to develop.
(Problem to be Solved By the Present Invention)
Lithium secondary batteries are used as power sources for portable telephones, small size personal computers and the like, however, when used for these applications, the total discharge capacity, total discharge efficiency, effective discharge capacity, and effective discharge ratio (these are called discharging characteristics) are insufficient, and improvements in these are desired.
Lithium secondary batteries, in general, have the problems of irreversible charging and discharging due to which the whole of the charging capacity cannot be discharged, and that the total discharge capacity and effective discharge capacity are low.
In addition, in a secondary battery which is used at a certain fixed voltage, large effective discharge capacity which can maintain that voltage, and large effective discharge ratio are sought, and conventional lithium secondary batteries do not have satisfactory discharging characteristics.
The present invention learning from the above circumstances, aims to provide a carbon for a lithium secondary battery which can be used in the manufacture of lithium secondary batteries which have excellent discharging characteristics by using this carbon in the electrode material of chargeable lithium secondary batteries, a manufacturing method therefor, and a lithium secondary battery.